Individual cancer patients were tested repetitively, at increasing postoperative intervals by a leukocyte migration test (LMT) and a skin window (SW) procedure. One quarter of 48 patients demonstrated LMT reactivity 1-8, 9-14 and 15-24 months post-operatively while one-fifth of the patients were minimally responsive during each of these intervals. Among 21 patients showing minimal reactivity to autologous breast cancer 1-8 months postoperatively, approximately half were reactive 9-14 and/or 15-24 months postoperatively. CMI to autologous breast cancer 9-24 months postoperatively was prognostically favorable regardless of the reactivity 1-8 months postoperatively. The phenomenon of spontaneous variations in CMI to autologous breast cancer suggests the possibility of experimentally maintaining or inducing such reactivity. Spontaneous variations in LMT reactivity to autologous breast cancer also provide a device for evaluating the specificity of such reactivity by simultaneously testing other targets. Among individual patients, tested repetitively 9-50 months postoperatively, reactivity to autologous breast cancer was correlated with reactivity to gp 55 of R III MuMTV and to its (gp 55) glycopeptide fraction. No such correlation was found with fetuin, its glycopeptide, benign breast lesions or colon carcinoma. Horizontal studies of simultaneous LMT reactivity to autologous breast cancer and selected targets provide important insights into the biological significance and specificity of such reactivity. Such information is pertinent to the development of immunotherapy and immunoprophylaxis and to the choice of patients for adjuvant chemotherapy.